1. Field of the Invention
This invention is related in general to the field of backup-battery systems. In particular, the invention consists of a system for staggering the charging of a plurality of backup-battery packs so as to reduce the number or size of the battery packs.
2. Description of the Prior Art
In FIG. 1, a traditional backup-battery system 10 includes a control device 12 having a power sensor 14, a charging circuit 16, and one or more battery packs 18. Each battery pack 18 includes one or more batteries 20 and has a maximum energy capacity based on the physical characteristics of the batteries. Once discharged below a predetermined discharge threshold, some battery packs are designed to be recharged to their maximum energy capacity. However, a battery pack's maximum energy capacity will deteriorate over time due to temperature fluctuations, repetitive charging/discharging, and other chemical reactions. For example, a typical battery may lose up to 20% of its maximum energy capacity over its useful life. This decrease is maximum energy capacity is more pronounced in some types of batteries if they are not fully discharged to their discharge threshold prior to being recharged.
If a battery pack is charged to its current maximum energy capacity, the charge will also deteriorate over time (“self-discharge”), even when not under load, due to chemical reactions within the battery. One approach is to provide a continuous charge (“trickle charge”) to the battery pack to prevent self-discharge. However, this creates other problems such as dramatically reduced battery life. Accordingly, it is desirable to have a backup-battery system to compensate for self-discharge without incurring the reduced battery-life penalty of trickle charging.
The energy available to power devices (“effective energy capacity”) is a function of the current maximum energy capacity, the current amount of self-discharge, the current temperature, and the load placed on the battery pack. This effective energy capacity is often significantly less than a battery pack's original maximum energy capacity. Accordingly, a backup-battery system must be designed to provide a sufficient effective energy capacity to power a load, even if the effective energy capacity is significantly lower than a battery pack's original maximum energy capacity.
Another approach is to utilize a battery pack with an initial maximum energy capacity significantly higher than that needed to power the intended load. However, increasing a backup-battery system's initial maximum energy capacity is costly and requires a larger volume of space for the correspondingly larger battery pack. Additionally, increasing the size of a battery pack can dramatically increase the weight of the backup-battery system.
Yet another approach is to utilize multiple battery packs with a combined initial maximum energy capacity significantly greater than necessary for the design load. Since the packs are in the same environment, they will tend to self-discharge at the same rate, potentially reaching the discharge threshold at approximately the same time. This solution suffers the same drawbacks as utilizing a single larger battery pack, i.e., increased cost, volume, and weight. Therefore, it is desirable to have a system for charging multiple backup-battery packs that prevents the battery packs from approaching the discharge threshold at the same time.
In U.S. Pat. No. 6,583,603, Baldwin discloses an apparatus for charging and discharging battery cells employed as a back-up power supply. The apparatus partially isolates battery cells from a load bus and power supply utilizing two control switches arranged in parallel. One of the control switches connects one set of batteries to the load bus at a time. However, while Baldwin discloses charging sets of batteries independent of other sets, he does not disclose preventing a plurality of battery packs from simultaneously approaching a discharge threshold.
U.S. Pat. No. 6,504,344, Adams et al. disclose a device for managing battery packs. A controller directs the selective load-testing of individual batteries. A battery whose useful capacity has fallen below a predetermined threshold is recharged. However, as with the Baldwin patent, Adams does not disclose alternating the charging of parallel battery packs to reduce the amount of excess design capacity. Accordingly, it is desirable to have a backup-battery system capable of staggering the charging of multiple battery packs to minimize the required initial maximum energy capacity.